1. Field of the Invention
The present invention relates to an exhaust gas purifying method for purifying exhaust gases emitted from an internal combustion engine by removing NOx contained therein.
2. Description of the Related Art
As internal combustion engines advantageous in reduction of fuel consumption, there are known lean burning internal combustion engines on which an air-fuel ratio is controlled to be on the lean side of the stoichiometric air-fuel ratio and direct injection internal combustion engines in which fuel is injected directly into combustion chambers for lean burning (hereinafter, referred to generally as lean-burn engines). In the lean-burn engine which runs leaner than stoichiometric (lean operation) to improve fuel economy, an exhaust gas purifying catalyst (a NOx trapping catalyst) is provided for purifying exhaust gases through reduction of NOx (oxides of nitrogen) therein.
The NOx trapping catalyst is made as a catalyst having characteristics in which NOx in exhaust gases is adsorbed as nitrate X—NO3 in an oxidation atmosphere (a lean air-fuel ratio) in which the concentration of reductants is low and the NOx so adsorbed is reduced to N2 in a reduction atmosphere (a stoichiometric air-fuel ratio or a rich air-fuel ratio) in which a large amount of reductants such as CO (carbon monoxide) and HC (hydrocarbons) exists.
In the NOx trapping catalyst of this type, NOx in exhaust gases is adsorbed to be prevented from being emitted into the atmosphere by the engine running with the lean air-fuel ratio, and the NOx so adsorbed is then desorbed for reduction by controlling the air-fuel ratio to be shifted to the rich side periodically. In order to obtain such a function, in the NOx trapping catalyst, for example, noble metals such as platinum (hereinafter, abbreviated to Pt), palladium (hereinafter, abbreviated to Pd), and rhodium (hereinafter, abbreviated to Rh), a NOx trapping agent such as an alkaline metal or alkaline earth metal and a heat-resistant inorganic oxide such as alumina are supported in a honeycomb substrate formed from a ceramic material.
In recent years, in the lean-burn engines, in order to exhibit the merit thereof in terms of fuel economy, efforts have been made to expand the lean operation range. Because of this, there are appearing some lean-burn engines which now run lean of stoichiometric even at the time of acceleration for which engines used to run with the stoichiometric air-fuel ratio or rich air-fuel ratio. Since fuel is consumed much when the vehicle is accelerated, the fuel economy can be improved largely by executing the lean operation at the time of acceleration.
In the current situations in which the lean operation range is expanded, there exists an increasing demand for improvement in the performance of NOx trapping catalysts, and a high NOx purifying performance at high temperatures is now being demanded. In related-art a NOx trapping catalyst, an alkaline metal or alkaline earth metal (a NOx trapping agent) is supported in a substrate formed from a ceramic material such as cordierite.
When the lean operation range is expanded, the temperature of the catalyst is increased, and the NOx adsorption material tends to migrate easily as the temperature of the catalyst increases. As this occurs, it is considered that the NOx trapping material combines with the constituent of cordierite. When the NOx trapping material is combined with the constituent of cordierite, the amount of NOx trapping material in the catalyst layers is reduced, leading to a fear that the reduction performance of the NOx is reduced. Because of this, it has been studied to apply to the substrate in which the catalyst layers are supported a metal substrate in which no combination of the NOx material occurs even at high temperatures.
In a metal substrate, a large number of cells are formed by winding corrugated foils and flat foils into a spiral configuration. Because of this, in a NOx trapping catalyst using such a metal substrate, corner portions having an acute angle are produced in one cell, and it is inevitable that the catalyst layer becomes thick at the acute-angled corner portions. When thick portions are distributed over the catalyst layer, there is caused a fear that the diffusivity of exhaust gases is reduced at the thick portions of the catalyst layers, and it has currently been difficult to increase sufficiently the exhaust gas purifying performance (in particular, with respect to NOx).
As a related-art NOx trapping catalyst, there has been proposed a NOx trapping catalyst which has a catalyst active component which contains an alkaline metal or alkaline earth metal and substrates and in which particles of magnesium oxide or calcium oxide are contained between the substrates (for example, refer to WO2002/62468). The related art described in WO2002/62468 is such that by the particles of magnesium oxide or calcium oxide being so contained, the migration of the alkaline metal or alkaline earth metal is prevented at high temperatures physically and structurally and configures a technique in which the thermal deterioration can be suppressed even when such metal substrates are used.